Laminated magnetic core for fluorescent ballast systems



March 3, 1953 c. A. HOWLETT 2,630,478

LAMINATED MAGNETIC CORE FOR FLUORESCENT BALLAST SYSTEMS Filed Dec. 6, 1950 2 b 32c, 3i] i, 23 4 3b 3a 5 /a K 2 K NI 2 Zc 3e 37; la i 1 2 Inventor'- ChaTles A. Howlett,

His Attow ney Patented Mar. 3, 1953 LAMINATED MAGNETIC CORE FOR FLUO- RESCENT BALLAST SYSTEMS Charles A. Hewlett, Fort Wayne, Ind., assignor to General Electric Company, a corporation of New York Application December 6, 1950, Serial No. 199,526

2 Claims.

This invention relates to laminated magnetic core structures and, more particularly, to those core structures which are used in ballasts for fluorescent lamps.

One of the more important requirements of a good fluorescent light ballast is that it be quiet in operation. oftentimes, noisy vibrations of the ballast case, or even of the fixture in which the ballast is placed, are caused by stray magnetic fields which leak away from the magnetic core or cores used in the ballast. Therefore, it is desirable to eliminate these stray fields as far as possible.

It has been known for some time that shelltype cores with center leg air gaps will give good results, since the center leg air gaps help reduce the amount of stray flux around any butt joints in the outside legs. Moreover, if the electrical winding or coils associated with the cores are positioned on the center leg so as to surround the air gaps, the coils will confine the stray flux within themselves and thus help prevent it from leaking into the case. This is easily accomplished in cores intended for use either in autotransformers or in reactors by building the core of layers of two E-shaped laminations, one at least having its center leg slightly shorter than the outside legs; thus, as the two outside legs are butted together, an air gap remains in the resultant center leg. However, for certain ballast applications, it is desirable to assemble an autotransformer and a reactor adjacent to each other on the same core with each having a distinct flux path. Up to this time, not satisfactory structure, other than one which mechanically, rather than magnetically, combines two separate cores, has been brought forward which combines the autotransformer and the reactor on the same core with each having its own distinct shell-type fiux path containing a center leg air gap, the center leg air gap being in such position that it can be surrounded by a coil.

Therefore, it is an object of this invention to provide an improved unitary laminated magnetic core structure which embodies two distinct shelltype flux paths, each flux path having a center leg air gap in such a position that it, the air gap, can be surrounded by a winding.

It is a further object of this invention to provide an improved fluorescent light ballast by using the above-mentioned core structure.

The way in which this invention fulfills these objects can best be understood by referring to the following description together with the accompanying drawing, in which Fig. 1 is a plan view of one layer of a preferred embodiment of this invention; Fig. 2 is a perspective view of a core, made from a plurality of the layers shown in Fig. 1, together with the windings associated with it; and Fig. 3 is a fluorescent lamp circuit diagram employing a ballast embodying the core and coils shown in Fig. -2.

Referring to Fig. 1, the lamination layer comprises two E-shaped laminations or 'punchings l and 2 and a roughly double-E-shaped lamination or punching 3. By double-E-shaped it is meant that punching -3 is roughly shaped in the form of two Es back-to-back. Punching 3 has center legs 3a. and 3b which are shorter than its outside legs 30, 3d, 3e and 3). The legs of the E-shaped laminations I and 2 are here shown as all of the same length, but the center legs la and 2a may be shorter than the outside legs lb, [0, 2b and 26,. if so desired. The outside legs of the E-shaped laminations l and 2 are butted against the out side legs 30, 3d, 36 and 3] of the double-E-shaped lamination 3, thus leaving air gaps 4 and 5 between the center legs 3a and 3b of the double-E- shaped lamination 2 and the center legs la and 2a. of the E-shaped laminations l and 2.

In Fig. 2, a number of layers formed as in Fig. 1 are stacked together to form a magnetic core structure 6. An autotransformer winding 5 is positioned on one center leg surrounding its air gap 4, while a reactor winding 8 is positioned on the other center leg surrounding its air gap 5. Each winding, thus, has its own distinct shelltype flux path, each path having separate legs and one separate yoke, but both paths sharing one common yoke, the common yoke being the stem of the double-E-shaped laminations.

Fig. 3 shows how the windings l and 8 may be connected in a fluorescent light ballast system. This particular system is composed of two fluorescent lamps 9 and Hi, the autotransformer winding 1 with a primary or common portion Ia and an extended portion 1b, the reactor winding 8 and two capacitors II and I2. Alternating electric power is supplied to the system through lines I3 and M, which are connected by switch I5 to a power supply. For safety reasons, connection is made to the autotransformer through the lamp sockets so that when either lamp is removed the autotransformer loses its excitation.

The operation of the circuit shown in Fig. 3 is as follows: Assume the switch [5 to have just been closed, thus energizing the system. Immediately the lamp it! will have the supply voltage plus the voltage of the winding portion lb impressed across it, since there is a closed path,

except for lamp ill, for alternating current flow from line l3 to line l4 through capacitors H and I2, the reactor 8 and the extended portion lb of the autotransformer l. Lamp it breaks down and current begins to flow. Capacitor H is of such magnitude that it now is the major impedance of the circuit, the voltage drop across it being but slightly less than the voltage originally impressed on lamp 10. Since capacitor H is in parallel with lamp 9, its voltage causes lamp 9 to break down and current passes through lamp 9. The circuit now assumes its steady state condition with current flowing from line 13 through lamp 9, capacitor I2, reactor 8, the autotransformer extended portion lb, and lamp it, all in series to line H.

The sequented starting of the lamps is very rapid and for all ordinary purposes both lamps start substantially simultaneously and instantaneously when switch [8 is closed.

The reactor 8 and the capacitor i2 constitute a net capacitive current limiting or ballasting series impedance for the lamps. The reactor 8 improves the lamp current and voltage wave shapes so as to increase the efiective lamp wattage and light output. It also has a stabilizing eiiect on the circuit with variations in supply voltage. The air gap 5 associated with the reactor 8 controls its reactance as to both magnitude and linearity.

The air gap 4 associated with the aut0transformer 7 increases its magnetizing current so as to compensate for the leading load current produced by the net capacitive impedance in series with the lamps and thereby achieve substantially unity input power factor for the system.

Although this circuit is not broadly new to the art, applicants novel core and coil structure, hereinbefore described, which combines the autotransformer 1 and the reactor 8 in a single electromagnetic unit of desirable characteristics in such a system, is believed to be novel.

While I have described only one preferred embodiment of my invention by way of illustration, many modifications will occur to those skilled in the art and it will, therefore, be understood that I intend by the appended claims to cover all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A laminated magnetic core structure formed to provide two distinct shell-type flux paths, each layer of said core being formed of three laminations, two being roughly E-shaped and one being roughly double E-shaped, the outside legs of said E-shaped laminations being butted against the outside legs of said double-E-shaped laminations, one center leg of a lamination in each pair of butted laminations being shorter than its outside legs to provide an air gap between said center legs of said double-E-shaped laminations and the center legs of said E-shaped laminations, each of the pairs of center legs between which there is an air gap constituting a separate winding leg portion of said core structure.

2. A fluorescent lamp ballast including a unitary laminated magnetic core having two distinct shell-type flux paths, each layer of said core having three laminations, two of said laminations being roughly E-shaped and one being roughly double-E-shaped in the form of two E's back-to-back, the outside legs of said E- shaped laminations butted against said outside legs of said double-E-shaped laminations, the center legs of said double-E-shaped laminations being slightly shorter than the outside legs, to provide two air gaps between said center legs of said double-E-shaped laminations and the center legs of said E-shaped laminations; an autotransformer winding positioned on a pair of opposed center legs surrounding a portion of each of said opposed center legs and the intervening air gap and a reactor winding positioned on the other pair of opposed center legs surrounding a portion of each of said other opposed center legs and the intervening air gap.

CHARLES A. HOWLETT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,803,868 Porter May 5, 1931 1,875,358 Sola Sept. 6, 1932 2,310,742 Ford Feb. 9, 1943 2,346,621 Sola Apr. 11, 1944 2,352,073 Boucher et al June 20, 1944 2,440,540 Farr Apr. 27, 1948 2,561,855 Gould July 24, 1951 FOREIGN PATENTS Number Country Date 606,024 France June 5, 1926 

